The composite products have great importance as cutting ceramics by virtue of their great toughness and their chip cutting action. Sintered composite products based on Al.sub.2 O.sub.3 with mechanically resistant materials embedded therein are the subject matter of DE-A 1 671 095, DE-A 2 471 295 and DE-A 3 529 265. Lee and Kim describe the production of Al.sub.2 O.sub.3 --TiC composite products by pressure-free sintering in J. M. Ceram. Soc., 72 (8), 1333-37 (1989). Al.sub.2 O.sub.3-- SiC composite products are disclosed in EP-A 0 311 289, FR-A 2 349 633 and FR-A 2 430 445.
According to EP-B 0 317 147, An Al.sub.2 O.sub.3 composite product is obtained by sintering suitable pulverulent starting materials at temperatures of from 1500.degree. C. to 1900.degree. C.
The system Al.sub.2 O.sub.3 --Al.sub.4 C.sub.3 --AlN and the formation of oxycarbides and oxynitrides of aluminum are described in J. Am. Ceram, Soc., 72 (9), 1704-709 (1989).
SiC-whisker-reinforced ceramics are the subject of a publication by Wei and Becher in Am. Ceram. Soc. Bull 64(2)298-304(1985).
Al.sub.2 O.sub.3 --B.sub.4 C sintered products are mentioned by K.C. Radford in J. Mater. Sci., 18, No. 3, 669-678/1983.
Materials in which carbonitrides (mixed phases of nitrides and carbides) are, inter alia, also incorporated in the Al.sub.2 O.sub.3 matrix are the subject matter of U.S. Pat. No. 4,320,203.
In U.S. Pat. No. 4,325,710 materials are described in which the matrix is doped with TiN, Zr and ZrC.
In U.S. Pat. No. 4,204,873, Yamamoto, Sakurai and Tanaka describe ceramic materials whose structure is strengthened by the incorporation of WC or W.sub.2 C in combination with other mechanically resistant materials.
In all these described processes, the starting materials are mixed in the form of their powders, the mixture is compressed and the compacted body is sintered at relatively high temperatures. In some cases, pressure and inert gas are also employed. The crystallite size of the Al.sub.2 O.sub.3 matrix obtained is in no case less than 1 micron.
The grinding performance of the ceramic obtained by this process does not meet the high standards currently demanded of modern abrasives.
Processes for obtaining composite Al.sub.2 O.sub.3 products via the melt have also been described. Thus intercalation compounds of Al.sub.4 O.sub.4 C and Al.sub.2 OC in an Al.sub.2 O.sub.3 matrix are obtained by melting and controlled cooling of a mixture of Al.sub.2 O.sub.3 with a carbon carrier (EP-A 0 022 420, AT-A379 979). A composite product containing oxynitrides as well as oxycarbides is described in FR-A 2 628 414. According to DE-C 152 501, a composite product of Al.sub.2 O.sub.3 and B.sub.4 C was already prepared in 1902 by melting in an electric resistance furnace. DE-B 1 049 290 describes an abrasive based on Al.sub.2 O.sub.3 with intercalated TiC, also obtained via the melt.
Compared with the sintered products described above, the Al.sub.2 O.sub.3 matrix in the melt-derived material has a substantially coarser crystalline structure.
The object of the present invention is the realization of abrasives which do not have the described disadvantages of the above-mentioned ceramic materials.